Starting today, their opinions don’t matter much. The radical operating-system upgrade and Surface, Microsoft‘s PC/tablet hybrid, are now on sale. Windows 8’s fate rests in the hands of millions of consumers, the vast majority of whom haven’t been paying much attention until now.

When people ask me how I think Windows 8 will do — which, lately, they’ve been doing approximately 15,000 times a day — I have a stock answer. And it’s an evasive one: I say that more than with any major product I can think of, it’s tough to figure out what’s going to happen with this one. I don’t think that Microsoft or PC makers or opinionated bloggers really know; there are too few parallels in PC history to compare this rollout with.

So rather than prognosticating about Windows 8’s future, I’m happy to be surprised by whatever happens. But I do have a few tips for anyone who wants to judge how it’s doing:

Don’t obsess over Windows sales figures. Windows 8 is Windows, so it’s going to ship on the vast majority of computers starting today, no matter what. So the fact that Microsoft is going to move a lot of copies isn’t in itself a sign that consumers are loving the upgrade. (Windows Vista may have turned out to be a failure, but it sold in impressively large numbers.)

Do pay attention to PC sales figures. They’ve been sluggish lately, and one theory holds that the slump is in part due to everyone biding time until Windows 8 shows up. If the next few quarters show improvement, it may mean that Windows 8 is getting consumers interested in PCs again.

Talk to friends and family. The ones who aren’t tech enthusiasts, I mean. Their reactions to Windows 8, pro or con, are going to do more to determine the product’s fate than any other single factor. It’s going to be fascinating to learn whether they love it, hate it or are just plain baffled by it — and to what degree they see the absence of the beloved Start button as a poke in the eye.

Look for killer apps. When Windows was young, back in the 1980s, it was applications such as Excel which convinced DOS users that this new and radically different environment was worth adopting. Windows 8 needs programs that are so compelling that vast numbers of people think to themselves: “Holy cow, I need to get this.”

Watch PC design. Hardware makers have done a pretty good job of putting the new operating system on computers designed for it — ones with touchscreens, detachable keyboards and other new features. At the same time, they’re continuing to make many, many garden-variety PCs. If consumers snap up the new-and-inventive machines, it’ll be better news for Windows 8 than if they’re inclined to stick with the conventional ones.

Monitor Surface’s success, or lack thereof. Surface is likely to represent a tiny fraction of overall Windows 8 sales, but it’s the Windows 8 hero device — the one Microsoft designed to be the ultimate expression of what it’s trying to do with this operating system. (Yes, I’m aware that Surface for Windows RT, the model that goes on sale today, is technically not a Windows 8 device. But it’s too confusing to maintain that it’s something altogether different.)

Microsoft is selling Surface directly to consumers, so the only way we’ll get hard numbers about its sales is if the company chooses to disclose them. (If it’s doing well, Microsoft is likely to share some stats; if it’s a dud, Microsoft is likely to stay mum.) But regardless of what Microsoft chooses to say, it’ll be possible to get a vague sense of how Surface is doing. If you frequently see people using it on airplanes and in coffee shops, that’s a promising sign; if you never seems to show up in the wild, that could be ominous.

Be patient. It’s possible that Windows 8 will start to look like a hit almost immediately. But even if it ends up being one, it’s more likely that it’s going to take a while until consumers and developers get their heads around all of its fundamental changes to an operating system which hadn’t seen much fundamental change in years. And even folks who are intrigued by the basic idea may think it’s smartest to sit back and see where it goes. If you think you’re going to be able to come to any definitive conclusions about Windows 8’s long-term prospects in 2012, you’re jumping the gun; the situation may be fuzzy even a year from now.

Heck, I believe that it’s conceivable that Windows 8 will look like a disappointment — but that the next major upgrade (Windows 8.5? Windows 9?) will be a winner.

Way back in June 2011, when Microsoft first previewed Windows 8’s altogether-new interface, I said “this is fun.” That’s still the word I associate with the new OS. As real people get their hands on it and form visceral reactions to what Redmond hath wrought, watching what happens is going to be a blast — and that’s the only prediction about Windows 8 I feel comfortable making.